1. Field of the Invention
The present invention relates to a decompressor and a vehicle, and in particular, it relates to an improvement in a decompressor of the type having a decompressor pin operating shaft in an oil passage provided in a valve cam shaft.
2. Description of the Related Art
Decompressors of engines force exhaust valves to open slightly in the process of compression at engine start-up or at engine stopping, thereby reducing the cranking torque of the engine to improve engine start-up performance. In other words, the decompressor releases a portion of compressed gas from a combustion chamber by slightly opening exhaust valves in the process of compression, thereby reducing cranking torque. Also, the decompressor slightly opens exhaust valves to reduce cranking torque during engine starting. The provision of such a decompressor to an engine allows an engine starter motor to be decreased in size and a battery for driving the starter motor to be reduced in capacity.
The patent document JP-A-2001-173421 discloses a structure of an engine decompressor, in which a valve cam has a pin hole on the surface, through which a decompressor pin is provided. A decompressor pin operating shaft is disposed in an oil passage provided in a valve cam shaft. The decompressor pin moves vertically with the rotation of the decompressor pin operating shaft. The decompressor pin operating shaft is rotated by a fly weight that rotates against the biasing force of a return spring, according to the rotation speed of the valve cam shaft. With the decompressor, when the fly weight rotates as the rotation speed of the valve cam shaft increases, the decompressor pin, whose head projects to the surface of the valve cam during engine stopping, extends into the deep pin hole thus achieving the above described decompressing function.
The decompressor disclosed in JP-A-2001-173421 has a decompressor pin operating shaft in an oil passage, thus allowing a smooth decompressing operation in normal environments, so that it may be advantageous. However, under very low temperatures, the viscosity of lubricating oil becomes extremely high, and the pressure of the lubricating oil significantly increases. In that case, the decompressor pin operating shaft is pushed axially in the valve cam shaft to bring the end surface into contact with another member, thus preventing smooth rotation of the decompressor pin operating shaft. Particularly, the fly weight is journaled at its rim, and the biasing force of the return spring is a relatively small value to operate the fly weight normally at a low rotation speed. Accordingly, when the decompressor pin operating shaft is pushed strongly against the center of the fly weight, even the motion of the fly weight may be locked.